The present invention relates generally to a photomask used for fabricating high-density integrated circuits such as LSIs and VLSIs and a photomask blank for fabricating such a photomask. The invention relates more particularly to a halftone phase shift photomask that enables a projected image of fine size to be obtained, a halftone phase shift photomask blank for fabricating such a phase shift photomask and methods for fabricating them.
Semiconductor integrated circuits such as ICs, LSIs, and VLSIs are fabricated by the repetition of the so-called lithography process using a photomask. To fabricate integrated circuits of fine size, the use of phase shift photomasks such as those set forth in JP-A 58-173744 and JP-B 62-59296 have been under investigation. Currently available are phase shift photomasks of various structures, among which is the halftone phase shift photomask (such as one set forth in U.S. Pat. No. 4,890,309) that is attracting attention due to its likelihood of being able to be immediately put to practical use. As disclosed in JP-A 5-2259, JP-A 5-127361, or the like, some proposals have been made in terms of construction and material that enable yield to be increased due to a reduction in the number of fabrication steps, and cost to be cut down.
A typical halftone phase shift photomask will be explained briefly with reference to FIGS. 3 and 4. FIG. 3 illustrates the principles of the halftone phase shift lithography, and FIG. 4 represents those of the prior art. FIGS. 3(a) and 4(a) are sectional schematics of photomasks; FIGS. 3(b) and 4(b) show the amplitude of light on photomasks. FIGS. 3(c) and 4(c) illustrate the amplitude of light on wafers; and FIGS. 3(d) and 4(d) depict the intensity of light on wafers. Reference numerals 101 and 201 stand for a substrate; 202 represents a 100% light blocking film; 102 indicates a semi-transparent film that effects a substantially 180.degree. phase shift of incident light and has a transmittance of 1% to 50%; and 103 and 203 represent incident light. In the prior art shown in FIG. 4(a), the 100% light blocking film 202 which, for example, is made up of chromium is merely formed on the substrate 201 which, for example, is made up of quartz glass, thereby defining a light-transmitting portion in a desired pattern using this arrangement there is a drop in resolution because the light on the wafer shows a divergent intensity distribution, as can be seen from FIG. 4(d). In the halftone phase shift lithography, on the other hand, the phase of the light passing through the semi-transparent film 102 is substantially inverted with respect to that of the light passing through the opening, so that, as can be seen from FIG. 3(d), the light intensity can be reduced to zero at the pattern boundary on the wafer or can be prevented from having a divergent distribution. Therefore there is an increase in resolution.
Here, a point worthy of mention is that in phase shift lithography processes of types other than the halftone type the lithographic step should be repeated at least twice because the pattern of the light blocking film differs from that of the phase shifter film, whereas in the halftone phase shift lithography the lithographic step need not essentially be repeated because there is one pattern. This is a great advantage of the halftone phase shift lithography.
In the halftone phase shift photomask, it is to be noted that the semi-transparent film 102 must serve a two functions phase inversion and transmittance control. In the phase inversion function, the phase of the exposure light passing through the halftone phase shift portion is substantially inverted with respect to that of the exposure light passing through the opening Now assume that the halftone phase shift layer 102 can be treated as an absorbing film described in, for instance, M. Born and E. Wolf, "Principles of Optics", pp. 628-632, and so multiple interference can be neglected. Then, the phase change of the vertically transmitted light is found by ##EQU1## When .phi. is within the range of n.pi..+-..pi./3 where n is an odd number, the phase shift effect is achievable. In Eq. (1);
.phi. is the phase change of the light transmitted vertically through a photomask having (m-2) layers on a substrate, PA1 .chi..sup.k,k+1 is the phase change taking place on the interface of the kth layer and the (k+1)th layer, PA1 uk and dk are the refractive index and thickness of the material forming the kth layer, respectively, and PA1 .lambda. is the wavelength of exposure light, with the proviso that the layer of k=1 is the substrate and the layer of k=m is air.
On the other hand, the exposure light transmittance of the halftone phase shift portion, at which the halftone phase shift effect is achievable, is determined by the size, area, location, configuration, and the like of the transferred pattern, and varies with the pattern. To obtain the effect mentioned above, it is a substantial requirement that the exposure light transmittance of the halftone phase shift portion fall within the range of the optimal transmittance (determined by the pattern) .+-. a few %. Usually, this optimal transmittance varies largely within a wide range of 1% to 50% depending on the transferred pattern, when the transmittance of the opening is deemed as 100%. In other words, halftone phase shift photomasks having a variety of transmittances are required to accommodate every pattern.
In fact, the phase inversion and transmittance control functions are determined by the complex indices of refraction (refractive index and extinction coefficient), and thicknesses of substrate material and the material forming the halftone phase shift film (each material forming each layer in the case of a multi-layer structure). More exactly, what is used as a halftone phase shift layer of a halftone phase shift photomask is material whose exposure light transmittance comes within the range of 1% to 50%, when it is formed on a substrate such that the phase difference found by Eq. (1) comes within the range given by n.pi..+-./3 (n=an odd number). Known materials with these characteristics are compounds such as oxides, nitrides, fluorides, carbides, and chlorides of chromium compounds, as disclosed in JP-A 5-127361. These compounds vary in the above complex index of refraction depending on their composition, texture, structure, and the like. Thus, if the material is formed into a proper film depending on the transmittance demanded, it is then possible to obtain a halftone phase shift photomask having a desired transmittance.
As can be understood from Eq, (1), however, once the material to form a halftone phase shift film is determined, the film thickness needed to have the phase inversion function is determined. In other words, it is impossible to control the transmittance of the halftone phase shift portion by varying the thickness of the halftone phase shift film. To fabricate halftone phase shift photomasks having a variety of exposure light transmittances while maintaining the phase inversion function, it is required to keep a variety of halftone phase shift film materials on hand. Another consideration has been given to a method in which a halftone phase shift film is made up of one layer serving as the phase inversion function, and one layer serving as the transmittance control function. The thickness of the latter is controlled to achieve the transmittance demanded. A serious problem with all these prior methods, however, is that mass fabrication is not easy, because a choice of halftone phase shift film material, the optimization of film thickness, etc., must be made individually depending on a variety of transmittance requirements.
The optimal transmittance is often determined by the simulation of a semiconductor fabrication process using masks on the basis of pattern size, area, location and configuration, and other factors. In most cases, however, it is difficult to find the optimal exposure light transmittance only by use of such simulation, because the semiconductor fabrication process has a multiplicity of parameters. Thus, after the fabrication of a halftone phase shift photomask, there is a need to alter the transmittance of the halftone phase shift portion. However, such an alteration has, up to this point, been impossible, and this is one of the grave problems in associated with halftone phase shift photomask fabrication.